A federal judge heard opening arguments today in a suit challenging Michigan’s ban on same-sex marriage, with the plaintiffs’ attorney countering the argument that limiting marriage to opposite-sex couples is in the best interest of children.

“No other group in society has to pass a parental competency test before they’re allowed to marry,” attorney Carole Stanyar said this morning in U.S. District Court in Detroit, according to Reuters. “We would like this to be the last trial in America where same-sex couples have to defend themselves.”

April DeBoer and Jayne Rowse are seeking to overturn the state constitutional amendment, approved by voters in 2004, that prohibits same-sex marriage, plus a state law that prevents them from adopting each other’s children.

Michigan’s attorney general has filed papers with the court saying heterosexual marriage is “uniquely suited to the rearing of children.” An expert witness for the plaintiffs, however, disputed that claim.

“Children of gay and lesbian individuals show no discernible differences in outcomes,” said David Brodzinsky, an expert on child welfare. “The parenting qualities of gays and lesbians are no different from heterosexual couples.”

Michigan assistant attorney general Joseph Potchen cross-examined Brodzinsky, who acknowledged that some studies show a higher breakup rate among same-sex couples. Another assistant attorney general for the state, Kristin Heyse, defended the ban on the basis that it was approved by Michigan voters. “This case is about one thing: the will of the people,” she said. “This was not the whim of a few.”

As the proceedings began this morning, several dozen protesters on both sides of the issue demonstrated peacefully outside the courthouse. The trial, before U.S. District Judge Bernard Friedman, is expected to last about two weeks, according to Reuters.